Weight / Fat loss

I don't pretend to understand this- I imagine that the majority of the short term fluctuations are purely to do with hydration?

78kg yesterday morning, 77.5kg this morning.

Seems like a lot- although I suppose half a litre against the volume of the whole body is fairly trivial

not really, that's fuck all difference. what have you eaten? when did you last take a dump? 500gms is nothing.